Polycarbonate resins are widely used for optical applications because of their excellent transparency, impact resistance, heat resistance, dimensional stability and workability. For example, various types of laser light optical disks have been researched, developed and marketed as high-density, large-capacity recording media. In particular, with the recent advent of the multimedia age, various techniques have begun to be developed to allow recording of large volumes of video data on optical disks. One of the technologies that has been proposed is a film surface side-incident optical disk wherein data is read from one side utilizing a protective film surface side opposite to the side of the polycarbonate substrate, and this has been described in several publications. These include Japanese Unexamined Patent Publication (Kokai) HEI No. 8-235638, “One-Sided 12 Gbyte Large-Capacity Optical Disk” (O plus E, Vol.20, No.2, p.183 (February, 1998)) and “Optical Disks and Peripheral Materials 98-2” (Summary of Lectures by the Japan Polymer and Optical Electronics Research Society, (Jan. 22, 1999)).
Referring to FIG. 1, such film surface side-incident techniques are characterized in that a data recording layer 2 consisting of a reflective film or recording film is formed on at least one side of a disk substrate 1 having a pattern of irregularities such as pits or grooves, on which there is further formed a thin film surface of about 0.1 mm thereover, and in that the film surface is formed by laminating a plastic film 4 onto the disk substrate through an adhesive layer 3. Data recording and reading is accomplished from the film surface side, and laser light focused through a lens 5 is applied onto the irregularity pattern.
When a transparent protective layer is adhered onto the optical disk substrate, it is considered preferable in terms of productivity to successively attach the plastic film which is to serve as the transparent protective layer onto the optical disk substrate, by pulling it out from a wound laminate of the plastic film. However, films intended for optical purposes which require extremely flat surfaces have poor slidability due to their high flatness, and normally cannot be wound into rolls as wound laminates.
Most conventional polycarbonate wound laminates for optical purposes are obtained by laminating an adhesive protective film onto a polycarbonate film. That is, an extremely flat plastic film for optical use is laminated by attachment of a protective film with a weakly adhesive side, whereby not only is that surface protected, but the structure of the opposite side of the protective film is also suitably rough to facilitate sliding so that the laminated film can be easily wound into long rolls. Such a protective film has a structure formed, for example, by coextrusion of polymers such as polyethylene and polyvinyl acetate, with the polyvinyl acetate side exhibiting adhesion for the polycarbonate film side. The films are attached together just prior to winding and gently nipped and taken up with a winder to produce a wound laminate.
As the step of forming a thin transparent protective layer on an optical disk, particularly using a polycarbonate film as the plastic film in a wound laminate, a polyethylene/polyvinyl acetate coextruded protective film was pulled out from a wound laminate, the protective film was released from the laminate and disk of the polycarbonate film was punched out and attached onto a polycarbonate optical disk. However, optical disks provided with thin transparent protective layers formed in such a manner have numerous irregularities (optical distortions) on the surface which are readily visible (although such surface irregularities are often invisible under ordinary light), and it has been confirmed that such surface irregularities (optical distortions) are a cause of major output fluctuations during signal input and output to and from the optical disk.
The present invention has been achieved in light of the circumstances described above, and its object is to provide a would laminate for a thin plastic film for data recording media which exhibits none of the problems alluded to above, allows large-capacity media to be obtained in an inexpensive manner, and offers greater convenience and industrial productivity, as well as an optical disk production process employing the wound laminate.